Kevin M. Lyons

Simultaneous Rayleigh imaging and CH-PLIF measurements in a lifted jet diffusion flame

Abstract Simultaneous Rayleigh scattering and CH planar laser-induced fluorescence (PLIF) measurements near the stabilization region of a lifted methane–air diffusion flame are presented. The goals of this investigation are to establish flow patterns responsible for complete breaks in the CH profile that indicate local flame extinction and evaluate the stabilization mechanisms over a range of flow conditions. Considerable attention has been given to vortex–flame interactions as a primary extinction mechanism of turbulent diffusion flames. The existence of holes in the flame zone is thought to result from the radial penetration of the flame by vortices from the internal fuel jet. In this investigation, Rayleigh scattering is used as a qualitative indication of gas temperature, thereby providing valuable information about the fluid near regions of local extinction, as indicated by well-defined breaks in the CH layer. The extent of premixedness in the region upstream from the CH structure is also assessed from the Rayleigh signal level. Furthermore, the roles of premixedness in flame stabilization, the nature of the leading edge, and lift-off height oscillation are discussed.

Simultaneous two-shot CH planar laser-induced fluorescence and particle image velocimetry measurements in lifted CH4/air diffusion flames

Joint two-shot CH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) measurements near the stabilization region of lifted methane/air diffusion flames stabilized under different flow conditions are presented. The simultaneous technique allows for a determination of the propagation rate of the CH zone relative to the fuel flow. Simultaneous single-shot CH-PLIF and PIV techniques have been used in the past to examine lifted jet flames: however, the double-shot technique of the current study is desirable because it yields information on flame dynamics—as indicated by sequential CH-PLIF—relative to the unburned mixture. Three flow conditions were examined corresponding to three different liftoff heights. While the average velocity at the stabilization point varies between 0.83 m/s for the lowest flow condition (Red=4800) and 1.28 m/s for the highest (Red=8300), the velocity at the stabilization point during instances of zero CH movement (during the time interval of the CH pulses) is constant for all three flow conditions (1.14±0.4 m/s). Furthermore, the flame is able to stabilize itself against the incoming unburned mixture only when the gas velocity is below a certain limit, above which the flame is convected downstream with the flow.

Studies on Lifted Jet Flames in Coflow: The Stabilization Mechanism in the Near- and Far-Fields

This paper presents the results of a parametric study concerning the phenomenon of liftoff of a nonpremixed jet flame. The dependence of liftoff height on jet exit velocity and coflow velocity is described. It is shown that lifted flames become less sensitive to jet exit velocity as the stabilization point recedes from the burner exit. The results reveal that in cases of extreme liftoff height, increases in jet exit velocity with a constant coflow cause some ethylene flames to stabilize closer to the burner. The success of current theories on lifted flame stabilization in comparison to the experimental results of this study are assessed. The existence of multiple regimes for flame stabilization, incorporating aspects of both premixed and nonpremixed combustion, is proposed.

Low reynolds number turbulent lifted flames in high co-flow

ABSTRACT This study presents the results of experiments designed to investigate flame lift-off behavior to nozzle velocity, co-flow velocity, fuel-type, and nozzle size for low Reynolds Number turbulent flows (in and near the hysteresis regime). Local excess jet velocities are computed using jet relations from Tieszen et al. The results show that the local excess jet velocity remains linear with respect to nozzle velocity through most of the hysteresis regime, even though flame lift-off height is not linear. This suggests a non-linear relation not captured by Kalghatgi (1984) for lift-off in the near field and hysteresis regime. Local excess jet velocities at the reattachment point were also computed for flames that are lifted more than three nozzle diameters above the burner. The results show that there is a minimum excess jet velocity for which a flame can stabilize. This minimum velocity is inversely proportional to the laminar burning velocity of the fuel squared. A new relation for lift-off height at the reattachment point for flames in the hysteresis region is derived and compared to experimental data.

Leading-Edge Flame Fluctuations in Lifted Turbulent Flames

Studies are presented that examine the fluctuations in liftoff height of lifted methane flames in the presence of air coflow. At a certain jet exit velocity, a flame will lift from the burner exit and stabilize at some downstream position. The partially-premixed flame front of the lifted flame oscillates in the axial direction, with the fluctuations becoming greater in flames stabilized further downstream. These fluctuations are also observed in flames where blowout is imminent. This work investigates the role of fuel velocity and air co-flow on flame fluctuations in both stable and unstable regimes. The results of video imaging of a lifted methane-air diffusion flame are presented and discussed. Images are used to ascertain the changes in the reaction zone that influence these fluctuations and relate the movement to blowout.

Leading-Edge Velocities and Lifted Methane Jet Flame Stability

Current interest exists in understanding reaction-zone dynamics and mechanisms with respect to how they counterpropagate against incoming reactants. Images of flame position and flow-field morphology are presented from flame chemiluminescence and particle image velocimetry (PIV) measurements. In the present study, PIV experiments were carried out to measure the methane jet lifted-flame flow-field velocities in the vicinity of the flame leading edge. Specifically, velocity fields within the high-temperature zone were examined in detail, which complements previous studies, whose prime focus is the flow-field upstream of the high-temperature boundary. PIV data is used not only to determine the velocities, but, along with chemiluminescence images, to also indicate the approximate location of the reaction zone (further supported by/through the leading-edge flame velocity distributions). The velocity results indirectly support the concept that the flame is anchored primarily through the mechanism of partially premixed flame propagation.

The effects of collector surface area with electrostatic flows resulting from multiple corona discharges

To initiate an ionic induced flow within atmospheric air, the high voltages are applied to the atmosphere by the coronas, creating an ionic flow to a grounded collector or ring. In experiments with a focus on the viability of applying the ionic wind as a cooling mechanism, using a shallow ring as the grounded collector for the ionized air, it was found that there is a relationship between the size of the grounded ring and the velocity of the flow caused by the ionization from the electrodes, maintaining voltage as a constant, a smaller ring provides higher efficiencies, while larger rings provide higher flow rates at larger dynamic pressures. Further research is being conducted on the influence of multiple corona discharges on the velocity and the effective static pressure of ionic flows in air as well as combustible flows

Effects of Diluents on Lifted Turbulent Methane and Ethylene Jet Flames

The effects of diluents on the liftoff of turbulent, partially premixed methane and ethylene jet flames for potential impact in industrial burner operation for multifuel operation have been investigated. Both fuel jets were diluted with nitrogen and argon in separate experiments, and the flame liftoff heights were compared for a variety of flow conditions. Methane flames have been shown to liftoff at lower jet velocities and reach blowout conditions much more rapidly than ethylene flames. Diluting ethylene and methane jets with nitrogen and argon, independently, resulted in varying trends for each fuel. At low dilution levels (∼5% by mole fraction), methane flames were lifted to similar heights, regardless of the diluent type; however, at higher dilution levels (∼10% by mole fraction) the argon diluent produced a flame which stabilized farther downstream. Ethylene jet flames proved to vary less in liftoff heights with respect to diluent type. Significant soot reduction with dilution is witnessed for both ethylene and methane flames, in that flame luminosity alteration occurs at the flame base at increasing levels of argon and nitrogen dilution. The increasing dilution levels also decreased the liftoff velocity of the fuel. Analysis showed little variance among liftoff heights in ethylene flames for the various inert diluents, while methane flames proved to be more sensitive to diluent type. This sensitivity is attributed to the more narrow limits of flammability of methane in comparison to ethylene, as well as the much higher flame speed of ethylene flames.

Lifting and Splitting of Nonpremixed Methane/Air Flames Due to Reactant Preheating

In order to assess the impact of initial reactant temperature on the occurrence of local extinction (LE) and the subsequent lifting process of non-premixed attached flames with increasing fuel injection velocity, hydroxyl radical planar laser-induced fluorescence (OH-PLIF) and high-speed CH*-chemiluminescence visualizations were conducted in a methane/air jet-flame, with preheating up to 1000 K. LE occurrence probability increases when approaching lifting, and the preheating level (Tox,ref) affects the probability density function (PDF) shape of LE axial origin. At low Tox,ref, partial lifting events occur near the burner lip, eventually leading the flame to lift directly from the very flame base. At higher Tox,ref, partial lifting events no longer occur, and LE is mostly witnessed in the flame breakpoint zone (axially from 1 to 3 jet diameters), resulting in a breakpoint lifting process. For very high Tox,ref (1000 K), local extinctions become widespread in the breakpoint zone so that a stable split flame is achieved prior to the lifted regime.

Experimental Observations of Nitrogen Diluted Ethylene and Methane Jet Flames

While the liftoff mechanisms of nitrogen-diluted methane jet flames have been well documented, higher order fuels, such as ethylene, have not been studied as extensively with regards to flame stabilization and behavior. Higher order fuels generally burn more intensely, and thus produce much different stabilization patterns than those of simple hydrocarbon fuels, such as methane. The purpose of this study was to observe the effects of nitrogen dilution on ethylene combustion and compare to that witnessed in typical methane jet flames; specifically, the influence on the liftoff height, blowout, and flame chemiluminescence. Liftoff and blowout velocities were compared for various mixtures of ethylene without nitrogen. It was observed that the reason behind the varying stabilization patterns is due to the higher thermal diffusivity of ethylene as well the higher flame speeds that are characterized in the combustion of ethylene. Using a sequence of images from each mixture, the flame liftoff heights were recorded. Due to the strong chemiluminescence of ethylene flames, little fluctuation between liftoff parameters was observed, with respect the velocity; however, there was a significant effect on the liftoff height, with respect to dilution. Blowout for fuel mixtures was much more difficult to achieve due to the higher thermal diffusivity of ethylene, meaning the flame would stabilize at positions much farther downstream than those of simple hydrocarbon fuels.Copyright